Hexafluoropropylene oxide is an intermediate which is used to prepare useful fluorine-containing compounds such as hexafluoroacetone and perfluorovinyl ether, as described in U.S. Pat. No. 4,165,340. Furthermore, polymers of HFPO have a wide variety of uses, e.g., a heat transfer medium and lubricant oil, as described in U.S. Pat. Nos. 3,660,315 and 3,775,439.
Hexafluoropropylene oxide can be prepared by epoxidization of hexafluoropropylene. It is, however, difficult to epoxidize hexafluoropropylene by the same method used in the epoxidization of propylene or allyl chloride since hexafluoropropylene has chemical properties which are very different from those of hydrocarbon olefins such as propylene or chlorinated hydrocarbon olefins such as allyl chloride.
For example, both propylene and allyl chloride can be epoxidized by the chlorohydrin process. In the process propylene or allyl chloride is first converted into chlorohydrin and, thereafter, ring closure is achieved by the use of alkalis. On the other hand, when the chlorohydrin process is used to epoxidize hexafluoropropylene, decomposition of the resulting chlorohydrin into carbonyl compounds occurs because of its instability, and it is therefore impossible to prepare the desired hexafluoropropylene oxide from the chlorohydrin.
Various methods for epoxidization of hexafluoropropylene have heretofore been proposed, which are different from those for hydrocarbon olefins and chlorinated hydrocarbon olefins. However, none of these methods can be used advantageously for the production of hexafluoropropylene oxide on a commercial scale.
Typical known methods of production of hexafluoropropylene oxide include the method described in U.S. Pat. 3,358,003 wherein hexafluoropropylene is oxidized into hexafluoropropylene oxide in a medium of alkaline hydrogen peroxide and the method described in U.S. Pat. No. 3,536,733 wherein hexafluoropropylene is oxidized into hexafluoropropylene oxide in the presence of an inert solvent. In accordance with these methods, however, it is not possible to obtain hexafluoropropylene oxide in good yields since the reaction is difficult to control, decomposition of hexafluoropropylene oxide formed is difficult to prevent, or large amounts of by-products are formed. Furthermore, when the HFP conversion is increased, the HFPO selectivity is lowered. Therefore, in order to efficiently use hexafluoropropylene, it is necessary to stop the reaction at low HFP conversions and to separate and recover unreacted hexafluoropropylene from hexafluoropropylene oxide for reuse. However, since the boiling point of hexafluoropropylene (-29.4.degree. C.) is very near that of hexafluoropropylene oxide (-27.4.degree. C.), it is difficult to separate them by distillation. Therefore, it is necessary to employ a specific separation technique. For this purpose, there has been proposed a method in which hexafluoropropylene is reacted with bromine in order to convert it into a high-boiling dibromo compound. This compound is then separated from hexafluoropropylene oxide as the dibromo compound. Another proposed method is an extractive distillation separation method as described in U.S. Pat. No. 3,326,780 and 4,134,796. These methods, however, are complicated and seriously increase the production cost of hexafluoropropylene oxide.
It is known, as described in IZV. AKAD. NAUK. SSSR. SER. KHIM., 79, (11) 2509, that hexafluoropropylene oxide is formed from hexafluoropropylene in a system comprising an aqueous hypochlorite solution with a polar solvent, e.g., acetonitrile and diglyme added thereto. Investigations of the oxidation method using such hypochlorites have revealed that the HFPO selectivity is about 10%. Thus it has been found that it is not possible to prepare hexafluoropropylene oxide in good yields. The reason for this is believed to be that since the reaction system is a homogeneous mixture of the polar solvent and the aqueous hypochlorite solution, the hexafluoropropylene oxide formed readily reacts with water under alkaline conditions, resulting in the decomposition thereof. Furthermore, this method requires an additional step to recover the polar solvent from the reaction system after the reaction. In view of the above described defects, the method is not suitable for practical use in the production of hexafluoropropylene oxide.